Many challenges have arisen as semiconductor device critical dimensions have decreased. In addition to scaling, contact critical dimensions have decreased due to the presence of non-planar geometries, such as fins and nanowires. Therefore, the thickness of barrier layers, such as TiN, is preferably reduced to allow sufficient metal conductor fill and provide reasonable metal conductor resistance. Barrier layers protect underlying metal from attack by precursors or other compounds that the underlying metal may be exposed to in the processing environment. Barrier layers may also be used to provide an adhesion layer.
Thinning of a barrier layer, however, allows plasma to penetrate into the underlying metal or semiconductor layer during the plasma densification process. Plasma penetration into the underlying layers may force elements from the barrier layer, such as nitrogen or oxygen, into the underlying layer. For example, in titanium/titanium nitride layers, plasma nitridization of the underlying titanium can occur. Presently, reduction of plasma nitridization of the underlying metal may be prevented by increasing the thickness of the barrier layer to the loss of performance in metal line resistance and trench pinch-off causing fill defects. Thus, room for improvement remains in plasma densification processes of barrier layers in semiconductor devices.